1. Field of the Invention
This invention relates generally to protective relay apparatus and methods for protecting ac electrical power transmission lines, and more specifically, to protective relay apparatus and methods providing single-pole tripping of a faulted phase conductor.
2. Description of the Prior Art
Three-phase ac electrical power transmission lines and power generating equipment must be protected against insulation faults and consequent short circuits or drops in shunt resistance that could cause collapse of the power system, serious and expensive apparatus damage, and personal injury. For instance, such a fault condition is caused by lighting-induced flashover from a transmission line to ground or between adjacent transmission line conductors. Under such a faulted condition, line currents can increase to several times the normal value, thereby causing loss of synchronism among generators and damaging or destroying both the transmission line and the attached equipment. To avoid equipment damage and collapse of the entire power system, faulted apparatus on the main transmission line must be isolated from the network in 0.1 to 0.5 seconds. The isolation time limit must allow for the operation of large circuit breakers interrupting up to 80,000 A and completion of back-up operations if these primary protective devices fail to function properly. To allow sufficient time for circuit interruption, location of the fault must be determined in approximately 8 ms to 20 ms. It is the function of the protective relays, which continuously monitor ac voltages and/or currents, to locate line faults and initiate isolation via tripping of the appropriate circuit breakers.
Most faults on three-phase ac electrical power transmission lines are transient single-phase-to-ground faults. Conventionally, such a fault is cleared by three-pole tripping followed by high-speed reclosing. Alternatively, the fault can be cleared by clearing only the faulted phase (i.e., single-pole tripping) followed by high speed reclosing. Use of single-pole tripping enables synchronizing power to be exchanged between the other two non-faulted phase conductors and minimizes system shock, ensuring a greater degree of stability. A large generation plant, for example, connected to the electrical power system via a single transmission line retains synchronization when single-pole tripping is used. Three-pole tripping requires the power generating machines to be resynchronized before reconnection to the system.
Two protective relays located at opposite bus terminals of an ac electrical power transmission line define a protected line segment. For single-pole tripping, the two protective relays, operating in communication with each other, must be able to distinguish a single-phase-to-ground fault from a phase-to-phase fault, a double-phase-to-ground fault, and three-phase faults, and must correctly identify the faulted phase. Several techniques are available for providing single-pole tripping, including: evaluation of the zero and negative sequence currents in a pilot relaying scheme; traveling-wave evaluation (see U.S. patent application Ser. No. 404,170, filed Aug. 2, 1982 now U.S. Pat. No. 4,438,475, and assigned of the assignee of the present invention), and segregated phase comparison.
As is well known in the art, to provide single-pole tripping with a phase comparison scheme, three system parameters must be identified by each protective relay and transmitted to the protective relay at the other end of the protected line segment. Thus, six modems are required. One novel feature of the present invention is the reduction of the required number of modems from six to four. This and other advantages of the present invention are discussed below in the DESCRIPTION OF THE PREFERRED EMBODIMENTS.